In a typical switch in a telecommunications system there is an interface between a two-wire transmission line and a four-wire transmission line. The two-wire line provides a connection between the telephone company's central office and the telephone company's customers. The four-wire line is part of the customer's local phone system. One of the functions of the telephone switch is to connect the telephone company's lines and the customer's phone system. Among other considerations, this involves matching the impedance of the customer's switch to the impedance of the telephone company's line. There are chips available which can be programmed to provide a specific impedance between two-wire and four-wire transmission lines. If the impedance of the telephone company's two-wire line is known, then this type of chip can be used to connect the two-wire line to the four-wire line with minimum signal loss and echo.
A limitation in this capability is that the expected line impedance may not equal the actual line impedance. As a result, the two-wire line impedance for which the customer's telephone system and switch are designed will not be correct and when the customer's switch is connected to the telephone company's two-wire transmission line there will be an impedance mismatch. In some cases there will be an echo on the four-wire side due to a mismatch in the impedances of the two-wire and four-wire lines. The mismatched impedance will often attenuate the telephone signals due to the inefficient power transfer across the transmission line interface. Generally, echo signals will also be caused by the mismatched impedance.
This problem may arise if a system is designed for the impedance of a specific two-wire line but it is installed instead on, or moved to, a different two-wire line. In some situations the line impedance may be measured incorrectly or it may be difficult to accurately measure the line impedance.
When a programmable matching chip is used, selecting the wrong impedance can cause other problems in addition to signal attenuation or echo. If the wrong two-wire line impedance is used to generate the matching filter coefficients, that could make the transfer characteristics of the matching circuit significantly worse than they would be if a nominal impedance value was used to generate the filters. On the other hand, use of a nominal impedance value may produce too simple of a matching circuit, thereby significantly distorting the transmitted signals. In some cases, the signal performance can degrade to such an extent that switch hardware, such as a modem, will not work when the wrong impedance is selected.